There is a broad class of measuring microscopes which project an image of a workpiece upon an aperture, and which measure properties of the light passing through this aperture, in order to characterize a particular small area, or a sequence of such areas, on the workpiece. We may describe this class as "aperture-projecting measuring microscopes."
One example of such an instrument is the microspectrophotometer, which characterizes the light spectroscopically. A microspectrophotometer which includes an illumination source for providing light to the workpiece, and means for determining the ratio of reflected light intensity to incident light intensity, as a function of wavelength, is called a microspectroreflectometer.
All aperture-projecting measuring microscopes require means to establish accurate focus of the microscope upon the selected area on the workpiece, so that the light passing through the aperture will correctly represent the properties of the selected area. In some cases it suffices for viewing means and large-field illumination to be provided, so that the instrument user can determine visually whether the instrument appears to be in focus. To increase the speed and reproducibility of measurements, it is preferable to provide automatic equipment to indicate when focus is correct, and in some cases to provide means for automatic adjustment of axial distance between the object and the microscope objective lens, so that the instrument is automatically driven to best focus.
When the workpiece to be measured has topography whose depth is comparable to or larger than the depth of field of the measuring microscope, it becomes particularly important that the focusing mechanism be responsive to the local surface altitude, in substantially the same region where the measurement is to be made. One group of workpieces that often exemplify this requirement are patterned semiconductor wafers used in the fabrication of integrated circuits.
The focusing systems to be described in this specification are suitable for use with aperture-projecting measuring microscopes. They are, in particular, suitable for use with microspectroreflectometers.
It is known that the best focusing height of a microscope may be determined by an apparatus in which the microscope objective projects upon the workpiece the image of a pointlike light source, and reimages the illuminated workpiece region on one or more pointlike apertures, behind which lie photoelectric detectors. Such an apparatus is described by Lacotte et al. in U.S. Pat. No. 3,912,922.
By "pointlike" is meant that the light source or aperture is smaller than the diffraction limit, so that the size and shape of images of the light source and aperture on the workpiece are determined primarily by the laws of diffraction, most detailed geometric information about the original shape being lost in the projected image. In the case of an essentially perfect and unobstructed microscope objective, the image is the wellknown Airy disc.
It has been shown possible to construct a profilometer (i.e. a measuring microscope for measuring the altitude profile of a surface) by employing such focussensing apparatus. Such a profilometer is described in D. K. Hamilton et al., "Surface Profile Measurement Using the Confocal Microscope", Journal of Applied Physics 53(7), 5320 (July, 1982), which is incorporated herein by reference. In such a profilometer, the focus sensor determines, at each of a series of points on the surface, the objective-to-stage distance that best maintains the focus of the objective on the surface; the record of the series of distance measurements represents the profile of the surface.
To understand how the focusing systems of our invention differ from other focusing systems, it is useful to review briefly some well-known principles of the confocal microscope. Such microscopes are described more fully in T. Wilson and C. Sheppard, Theory and Practice of Scanning Optical Microscopy, Academic Press, 1984, which is incorporated herein by reference.
FIG. 1 shows schematically a simple and common form of confocal microscope. Laser 102 produces a beam of light, which is brought to a focus by lens 104 on pinhole 106. Pinhole 106 is small enough to be substantially smaller than the diffraction limit for this optical system, so that laser light coming through the pinhole is effectively a point source. Condenser lens 108, which must be of high optical quality (often a microscope objective is used for this function) forms an image of pinhole 106 on the transparent object which is to be observed. The object (not shown) lies in object plane 110, and may be moved transversely to the optical axis of the instrument, so as to measure a profile of transmissivity vs. position. This instrument is, in other words, a form of microdensitometer. Objective lens 112, which typically has the same numerical aperture as condenser 108, forms an image of the illuminated spot on detector pinhole 114, behind which lies detector 116. Detector pinhole 114 is smaller than the diffraction limit.
Other known variations of the confocal microscope provide for building up a map or an image of an object not by moving the object, but rather by moving optical elements such as lenses or mirrors, so as to cause the observed spot to move. For simplicity in presentation, our invention will be described with respect to moving-object microscopes, but the moving-optics variations are also contemplated.
The confocal microscope provides better spatial resolution than does a conventional microscope. This point is illustrated by FIG. 2, which is the graph of the radial distribution of intensity that would be observed by the FIG. 1 microscope, as it scanned across a pointlike object.
Curve 201 is the intensity distribution that would be observed if either pinhole 106 or pinhole 114 were absent. This curve is just the well-known Airy intensity distribution that is observed with a conventional scanning microscope.
Curve 202 is the distribution observed with both pinholes in place. The observable enhancement in resolution is explained by the fact that the resolution is a product of two Airy-disc images. As the point of measurement moves away from the center of the actual pointlike object, the intensity of illumination falls off according to curve 201, and the sensitivity of the detector also falls off according to curve 201. The net sensitivity curve 202 is the product of these two curves.
Curve 203, representing the sensitivity of a confocal microscope with annular apodizing apertures (not shown) inserted in the pupils of each of lenses 108 and 112, is shown as one example of the fact that more elaborate versions of the confocal microscope can have even higher resolution, typically at the expense of some residual sensitivity at large distances from the center of the pattern. Thus the central portion 203a of curve 203 is narrower than the central portions of curves 201 or 202, but the sensitivity in rings 203b and 203c is higher than anything seen with curves 201 and 202. The effect on imaging is that resolution improves at the expense of introducing more artifacts in the image. For simplicity, our invention is described without the presence of annular apodizing apertures. We contemplate, however, the optional use of such apertures, or of more complex apodizing apertures.
FIG. 3 illustrates a second known property of the confocal microscope which is of importance in understanding our invention. This figure shows just the detection half of a system like that of FIG. 1, in three different conditions of focus. With reference to FIG. 3a, object 14 is imaged by microscope objective 40 onto an image plane 90 coinciding with aperture 46 in field stop 44. This condition occurs when object 14 is "in focus" wit respect to aperture 46. Note that the bundle of light rays 88 originating from a pointlike region on object 14 lying on a focal plane 86 comes to a focus at an image plane 90. All of these rays pass through aperture 46. In FIG. 3b, the object 14 lies below focal plane 86, and the image plane 90 is located below stop 44. Only some of the light rays in bundle 88 pass through aperture 46. Likewise, in FIG. 3c, object 14 lies above focal plane 86, and the image plane 90 again does not coincide with stop 44. Only some of the light rays in bundle 88 can pass through aperture 46. This results in the condition shown in FIG. 4a, where the intensity I of light passing through the aperture is at a maximum value I.sub.0 when the axial position Z of the object coincides with the focal plane position Z.sub.0.
This variation of the confocal microscope's response with focus condition is the basis of profilometers such as that described in the Hamilton et al. article cited above.
It is possible to construct a confocal microscope to work in reflective mode. In such an instrument, one form of which is illustrated schematically in FIG. 5, a single lens 510 acts both as condenser and objective. Laser 502 emits a beam of light which is focused by lens 504 on source aperture 506. Light which passes through aperture 506 then passes through beamsplitter 508, and is focused by objective 510 on a workpiece (not shown), which usually lies in object plane 512. Light reflected or scattered from the workpiece is gathered by objective 510, and focused, via beamsplitter 508, on detector aperture 514, behind which is located detector 516. The operation of the reflection mode confocal microscope is similar to that of the transmission confocal microscope previously described.
It is also possible to construct a reflection confocal microscope in which a single aperture is shared by the illumination and detection systems. One example of such a configuration is shown in FIG. 6. An instrument according to FIG. 6 is described in L. Reimer et al., "Lock-In Technique For Depth-Profiling and Magnetooptical Kerr Effect Imaging in Scanning Optical Microscopy", Scanning 9, 17-25 (1987). A particular advantage of such an arrangement is that no precision pinhole alignment is required to attain optimal performance. Whereas in two-pinhole instruments, the source pinhole's image must be accurately positioned in relation to the second pinhole, the single-pinhole system is automatically in alignment at all times.
Although confocal microscope configurations are usually described to include lasers, use of a laser is not in general strictly necessary. The laser is employed because it is an unusually bright light source, permitting high speed measurements with excellent signal to noise ratio. In cases where achieving maximum brightness is not the dominant consideration, it may be advantageous to construct a confocal microscope with an incoherent light source such as a tungsten lamp or an arc lamp. Most of the embodiments of our invention, to be described below, do in fact use incoherent light sources.
We have recognized that, while confocal microscopes have previously been described to use apertures whose size is less than the diffraction limited optical spot at each aperture, it is possible and sometimes advantageous to use larger apertures. In the aperture-projecting measuring microscopes considered in our invention, the size of the aperture is determined by the size of the workpiece area to be characterized, which is often larger than the diffraction-limited spot. We will explain below that there is considerable advantage in constructing a focus-sensing apparatus for use in such instruments by using confocal microscope configurations or our own inventive configuration, called the inverse confocal microscope.
Many aperture-projecting measuring microscopes suitable for modification to include our inventive focusing means have been described in the literature. For example, in U.S. Pat. No. 4,674,883, Baurschmidt discloses a microspectroreflectometer for measuring the thickness and line width of features upon an object, such as thin film structures on a semiconductor wafer. The Baurschmidt microscope incorporates no provision for detecting or automatically adjusting to a condition of best focus.
The accuracy with which the thickness of transparent films at specified locations on wafers or other flat surfaces can be measured is limited by the accuracy of focus of the microscope-spectrometer, which affects not only the resolution of the location on the wafer being measured, but also the amount of light reaching the spectrometer's detector elements. For very thin films, the measurement accuracy may also be affected by both the spectral resolution and the spectral range of the spectrometer. Measuring microscopes currently available can accurately measure the film thickness on unpatterned wafers down to about 10 nanometers. In the case of patterned wafers and other somewhat flat objects having a rough or profiled surface, either a large depth of focus is required or the microscope must be able to bring areas of the surface into focus as the object is scanned.
It is an object of the present invention to provide a measuring microscope capable of automatically focusing on an object as that object is scanned.
It is another object of the present invention to provide a microscope capable of accurately measuring characteristics of a workpiece area, such as the thickness of thin films on patterned wafers for thicknesses in a range from less than 2 nanometers to more than 5000 nanometers.